In the treatment of waste waters and especially as relates to the waste water treatment process referred to as "activated sludge systems" activated carbon is known to be beneficial. In the activated sludge system there is a requirement to remove from time to time or on a continuous basis some of the solids in the stream commonly referred to as "waste activated sludges". Such sludges contain powdered activated carbon and inert materials referred to as ash which must be removed from the system to prevent excessive build up of total solids in the process. There is also a need to reactivate the powdered carbon so that it can be reused. The powdered carbon and/or other forms of activated carbon can be reactivated and made suitable for reuse in a regeneration process known in the industry as "Wet Air Oxidation" (oxidation using oxygen either from the air or pure oxygen and high temperature without the use of flame induced heat). Under proper operating conditions the regeneration system proceeds in a wet oxidation reactor autothermal with operating temperatures 400.degree.-550.degree. F. and operating pressures 350 psig-900 psig. Under certain operating conditions the wet air oxidation reactor blow down that settles out from the bottom of the reactor along with some carbon is blown off into a "tank" commonly referred to as a "blow down tank". From the blow down tank the ash and some carbon is removed by flushing using water which mixes in with the blow down water, ash and carbon (to form a more dilute sludge). Ash must be removed from this mixture to eliminate build up of the inert ash in the activated sludge system.
Also, in the wet air oxidation process there is a continuous flow stream leaving the wet air oxidation reactor or system taking large quantities of reactivated carbon and some ash back to the treatment system. This is a continuous operation whenever the reactor and/or regeneration system is in operation. It is desirable to limit the amount of ash returning to the system, which would require separation of powdered carbon and water completely or partially from the ash so as to produce a cake or concentrated ash solid that can be handled and disposed of as an inert sterile solid on the property or in any other land disposal system available to the operation.
Another process requiring separation of ash is the controlled flame incineration operating at and under a controlled atmosphere which would limit the oxygen so as to produce ash but reactivate the carbon. In this latter process the hot burning ash-carbon mixture is dumped or introduced into a water quench tank and this resulting mixture requires separation of the carbon, ash and water. The processes also requires the carbon to remain in the liquid mixture and be returned to the process or in some cases to be returned to a tank for storage and reuse.
In all cases it is cost effective to remove the maximum amount of water from the ash so as to facilitate the disposal of the ash. This ash is inert and sterilized and does not present a problem for disposal and handling as long as the water content is low so as to have a mixture that would not be in a fluidized state that would tend to flow out of the area selected for disposal.
One known and used system for separating the powdered carbon portion from the ash uses gravity settling devices which, by regulation of the recirculation rate, the powdered carbon with water can be removed and taken back to the system for reuse with the heavier ash settling for removal of the ash and water from settling tanks. This system can have certain deficiencies in that the specific gravity and the temperature conditions can vary to such extent that there is not a satisfactory or economical separation and recovery of the carbon. One of the problems generally associated with this type of removal system is that the ash and some carbon forms a very dense, firm, cake on the bottom of the tank which creates a problem in removal. For removal, this cake or heavy type of sludge mixture has to be flushed out using additional water which fluidizes the cake and sludge resulting in a requirement for further separation and removal of excess water to facilitate and allow disposal of the ash on to the disposal site or ground. If the water is not removed then disposal is a problem. Furthermore, in the tank settling system considerable valuable activated carbon is mixed in and removed, resulting in valuable material being thrown away and lost. In connection with the tank settling system, it is known that temperature changes result in change in the settling or separation of carbon; therefore, changes in temperature of the water in the tank, changes in the amount of blow down and temperature of the cooled blow down water carbon ash mixture results in a variable process which is difficult to control and requires considerable time of personnel for operation of the system.